[Eeglablist] Electrode coordinates

Robert Oostenveld roberto at smi.auc.dk
Fri Mar 11 07:09:30 PST 2005


Hi Miguel

If the EEG is recorded using bipolar electrode pairs, you will not 
(easily) be able to plot the topographic distribution, which is where 
you mainly would need the channel locations for. The topoplot function 
shows the potential distribution of ERP/ICA components on all channels, 
where all channels are referenced identically. Improrant is that 
potential is not a number on itself, but it always is a potential 
difference, i.e. it refers to two electrodes (at least). The common 
channel to which all scalp electrodes are referenced towards in the 
topoplot can be one of the scalp electrodes (e.g. behind the ear), but 
it also can be the average of both ears, or the average of all 
electrodes.

You should think of the topographic display of potential as a 
geometrical height map: in general that will be expressed with respect 
to sea level.
In your case you only have the height differences between points, but 
you don't have the heigh at each point with respect to the sea level. 
This also explains the trick that you might be able to use for 
rereferencing the data to a common electrode: add and subtract all the 
height differences, untill you have expressed each of them with respect 
to a common "sea level".

For ICA decomposition, and for looking at the timecourses or frequency 
contents of your EEG signals, it does not really matter that they are 
bipolar, although the signal content of a bipolar channel from two 
neighbouring electrodes will be different than the signal from an 
electrode that is referenced to another electrode on the other side of 
the head.

best regards,
Robert

On 9 Mar 2005, at 16:10, Miguel Sota Zuñiga wrote:

>
> Hi all:
>  
> I have a eeg data imported from a text file, which was recorded in 
> columns like this:
>  
> "Time offset", "Fp1T3", "C3T3", "O1T3", "Fp2T4", "C4T4", "O2T4", 
> "C3A2", "C4A1", "Fp1C3", "C3O1", "Fp2C4", "C4O2", "Fp2Fp1d"
>  
> Each data was acquired from bilopar electrodes, for instances, Fp1T3 
> means measured between standard position Fp1 and T3. Notice that Fp2 
> is repeated 3 times.  
>  
> I´ll appreciate if someone could help me to know the coordinates 
> (polar, xyz, .. whatever) of this positions which I would use in 
> EEGLab readlocs function.
>  
> Thanks
> Miguel Sota
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----------------------------------------------------------------------
Robert Oostenveld, PhD
F.C. Donders Centre for Cognitive Neuroimaging
Radboud University Nijmegen
phone: +31-24-3619695
http://www.ru.nl/fcdonders/
----------------------------------------------------------------------
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